Because copper atoms diffuse rapidly into other materials, such as dielectric materials, conductors made of copper or copper alloy in a semiconductor device are surrounded with diffusion barriers. Prior diffusion barriers were formed by etching a trench or via in a dielectric substrate. Metallic compounds were then deposited by physical or chemical vapor deposition onto the surfaces of the substrate and the sidewall surfaces of the trench or via. These sidewall diffusion barriers formed of metallic compounds, however, were problematic for numerous reasons. First, most of the metallic compounds used have a polycrystalline structure through which atoms readily diffuse. Accordingly, impurities such as oxygen or nitrogen are incorporated into the metallic compounds in order to block these diffusion paths. The result is a metallic diffusion barrier with increased resistance due to the presence of the impurities. In addition, because metallic diffusion barriers occupy a significant portion of current-conducting area and have high resistivity (i.e., the resistivity ratio of metallic barrier material to copper is typically larger than ten (10)), the metallic diffusion barriers can produce a large increase in resistance-capacitance (RC) delay times. Consequently, the performance of a circuit formed with metallic sidewall diffusion barriers is degraded by the inherent RC characteristics of the barriers.